It’d take a pretty big crisis to gather the likes of Arrow, The Flash, Legends of Tomorrow and Supergirl (from another universe), leaving The CW to assemble a pretty major threat. And while the big four-way crossover remains well under wraps, it seems one major threat will unite the four, rather than different big bads for each installment.

I’m going to probably try to have one unifying threat, as opposed to different things on different ones, which is what the first year was.

Prevailing theories have suggested that the four-way teamup might somehow bring Supergirl’s Earth into the known Arrow-verse, if only to facilitate easier crossovers, but it remains to be seen if any one villain or specific crisis could facilitate that. In any case, we’ve already seen a brief tease of crossover, if only to enjoy Stephen Amellposing on set withTyler Hoechlin’s Superman.

We’ll find out much more in the coming months, but what one threat might unite the likes of Arrow, The Flash, Legends of Tomorrow and Supergirl?